The Advanced Television Systems Committee (ATSC) published a Digital Television Standard in 1995 as Document A/53, hereinafter referred to simply as “A/53” for sake of brevity. ATSC published “ATSC Mobile DTV Standard, Parts 1-8” on 26 Oct. 2009 as Document A/153, hereinafter referred to simply as “A/153” for sake of brevity. A/153 specifies robust ancillary transmissions time-division multiplexed into 8VSB DTV, which ancillary signals are designed for reception by mobile receivers and by hand-held receivers that are referred to collectively as “M/H receivers”. The ancillary data employ internet-protocol (IP) transport streams. The ancillary data are randomized and subjected to transverse Reed-Solomon (TRS) forward-error-correction (FEC) coding before serially concatenated convolutional coding (SCCC) that uses the 2/3 trellis coding of 8VSB as inner convolutional coding. This TRS FEC coding helps overcome temporary fading in which received signal strength momentarily falls below that needed for successful reception. The strongest TRS codes prescribed by A/153 can overcome such drop-outs in received signal strength that are as long as four tenths of a second.
Rows of data bytes and rows of parity bytes in the RS Frames of TRS-coded data are subjected to cyclic-redundancy-check (CRC) coding before SCCC. An M/H receiver can use the CRC coding as an error-locating code for the TRS FEC codewords. This permits the use of a Reed-Solomon decoding algorithm that can correct twice as many byte errors in each TRS codeword as an algorithm that must locate, as well as correct, byte errors. The SCCC coding is designed primarily for correcting errors arising from all-white-Gaussian noise (AWGN) or similar noise, and the TRS FEC coding is relied on for the correction of errors arising from sustained burst noise. The TRS FEC coding is quite effective in overcoming drop-outs in received signal strength that cause sustained burst noise leading to the loss or severe corruption of as much as two complete M/H sub-Frames of the received signal. There are five sub-Frames in each 986 milliseconds long M/H Frame.
The capability of the TRS FEC coding to correct frequently occurring shorter noise bursts is compromised, however, by the symbol interleaving employed between the outer convolutional coding and inner convolutional coding of the SCCC. This symbol interleaving disperses a noise burst occurring in a single data segment over several data segments that share the same Block of symbol interleaving. This tends to increase the number of rows in the RS Frame in which the CRC codes indicate byte error. When the TRS codewords all use the same CRC codes for error location, a modest sprinkling of short burst errors throughout the RS Frame may overwhelm the two-dimensional FEC decoding. The TRS decoders can be designed to change to an error-locating-and-error-correcting FEC decoding algorithm when this occurs. However, the performance of such an algorithm will also be compromised by the dispersal of burst errors by the symbol interleaving employed between the outer convolutional coding and inner convolutional coding of the SCCC. Such dispersal will cause more byte errors in TRS FEC codewords.
The symbol interleaving that follows outer convolutional coding in the M/H transmitter constructed in accordance with A/153 has two functions. It introduces time diversity between the single-phase outer convolutional coding and the inner convolutional coding afforded by the 12-phase trellis decoder, which is the sine qua non of SCCC. The symbol interleaving also compensates for complementary symbol de-interleaving in the M/H receiver. The symbol de-interleaving in the M/H receiver disperses lateral burst noise that accompanies the received SCCC and affects inner convolutional coding. Hopefully, the outer convolutional decoding will then be better able to overcome fragments of that lateral burst noise as so dispersed. The inventor noted that neither of these functions is affected by the order in which data is supplied for outer convolutional coding in the transmitter.
This validated his speculation that the dispersal of burst noise by the symbol de-interleaving in the M/H receiver might be overcome if the randomized M/H data were de-interleaved before outer convolutional coding of them. That is, so as to compensate against the subsequent symbol interleaving of the outer convolutional coding before inner convolutional coding. The inventor contemplated the M/H receiver being modified to interleave the randomized de-interleaved M/H data that would be recovered by the decoder for the outer convolutional coding before making hard decisions concerning the randomized M/H data. Since symbol interleaving of the results of decoding of the outer convolutional coding is customary in turbo decoding procedures anyway, the M/H receiver would require no substantial increase in size or complexity, the inventor perceived. The only change in receiver design would be an insightful relocation of the point in the turbo-decoding loop from which to extract input signal for the hard-decision unit.
The output signal from the hard-decision unit in a M/H receiver modified as described in the preceding paragraph comprises randomized IP transport stream (TS) packets that are written to rows of a framestore used for decoding the two-dimensional RS-CRC coding. Since these randomized IP transport streams are collateral with burst noise, fewer of them will contain burst noise for correction by the TRS FEC decoding procedures. Accordingly, if TRS FEC coding is unable to correct byte errors, it is likely that fewer IP TS packets will contain byte error than was the case with transmissions as specified by A/153.
The principal design task for the transverse Reed-Solomon (TRS) coding used in the RS Frames prescribed by A/153 is overcoming drop-outs in received strength caused by reception nulls when the receiver is moved through an electromagnetic field subject to multipath reception. However, the shortened 255-byte Reed-Solomon (RS) codes used for TRS coding are very powerful codes for correcting shorter burst errors, especially when used together with codes for locating byte errors. If RS codes are relieved of having to locate byte errors as well as correct them, RS codes can correct as many byte errors within each of them as each has parity bytes. If RS codes have to locate byte errors as well as correct them, they can correct only one-half as many byte errors within each of them as each has parity bytes. Providing a sufficient number of parity bytes in each RS code to implement the principal design task for TRS coding requires a significant investment in reduced M/H payload. So, care should be taken to maximize the return from that investment.
A/153 prescribes two-dimensional coding of RS Frames of randomized M/H data in which the bytes in each RS frame are cyclically redundantly coded row by row to form respective cyclical redundant code (CRC) codewords. These row-long CRC codewords can be used as error-locating codes for the TRS codewords, but only in common, on a collectively shared basis. This works reasonably well when overcoming protracted drop-outs in received strength caused by reception nulls when the receiver is moved through an electromagnetic field subject to multipath reception. These protracted errors typically extend over several rows of bytes in the RS Frame and affect all TRS codewords in the RS Frame.
Each occurrence of shorter burst noise is apt to affect only some of the TRS codewords in the RS Frame. Several occurrences of such shorter burst noise are apt to occur in some RS Frames. The row-long CRC codewords will respond to each occurrence of shorter burst noise to locate a byte error in every one of the TRS codewords in the RS Frame. Several occurrences of shorter burst noise in an RS Frame can cause the row-long CRC codewords to locate more possible byte-error locations than can be accommodated by a TRS decoder using a byte-error-correction-only decoding algorithm for correcting TRS codewords. The TRS decoder can be designed so as then to switch over to a byte-error-location-and-correction decoding algorithm for correcting TRS codewords. However, as noted above, the byte-error-correction capability of the TRS decoder is halved by switching over to a byte-error-location-and-correction decoding algorithm.
Using shorter cyclical redundant coding (CRC) codewords in each row of the RS Frame is likely to result in fewer TRS codewords requiring the switch-over to a decoding algorithm that provides both location and correction of erroneous bytes. If the RS Frame is coded in a number 5M of M/H Groups, M being an integer more than one, each row of bytes in the RS Frame is preferably apportioned into M or a prescribed multiple of M CRC codewords. These shorter RS codewords have utility in improving turbo decoding of the concatenated convolutional coding (CCC) for M/H receivers, particularly when parallel concatenated convolutional coding (PCCC) is used rather than the serial concatenated convolutional coding (SCCC) prescribed by A/153. PCCC transmissions at code rate one-half the 8-VSB symbol rate are preferred for iterative-diversity and frequency-diversity reception, as described by A. L. R. Limberg in his patent application Ser. No. 12/580,534 filed on 16 Oct. 2009 and titled “Digital Television Systems Employing Concatenated Convolutional Coded Data”.
CRC codes can be used to check whether or not strings of data bits in the results of decoding outer convolutional coding of a PCCC transmission at code rate one-half the 8-VSB symbol rate are presumably correct. Those strings of data bits with checksums indicating they are very likely to be correct can have the confidence levels associated with their parent soft bits heightened. Re-interleaving will scatter the parent soft bits descriptive of data that have the heightened confidence levels throughout the extrinsic information fed back via the turbo loop, to be used in the next iteration of decoding of inner convolutional coding by a turbo decoder. When the CRC codes indicate that substantially all the strings of data bits in the results of decoding outer convolutional coding of the PCCC transmission are very likely to be correct, this information can be used to discontinue the iterative procedures associated with turbo decoding the PCCC.
Suggestions have been made by some ATSC members to include further FEC coding of the IP transport stream to correct remnant errors in the rows of bytes from RS Frames supplied to later stages of the M/H receiver: The inventor observes that the problem with the proposed further FEC coding is that it is not transverse to the direction of the running errors that characterize the decoding of the outer convolutional coding of CCC when symbol interleaving is done per A/153. There is no interleaving to break up running errors insofar as the further FEC coding of the IP transport stream is concerned.